1. Field of the Invention
The present invention relates to a surface acoustic wave filter, and more particularly, to a surface acoustic wave filter including a glass substrate.
2. Description of the Related Art
Surface acoustic wave filters (SAW filters) have been widely used in various frequency ranges to define a bandpass filter or other such components. Such surface acoustic wave filters generally have a structure in which an interdigital transducer is disposed on a piezoelectric substrate. A surface acoustic wave filter in which an interdigital transducer is disposed between a piezoelectric film and a glass substrate is also known and is disclosed in, for example, Japanese Unexamined Patent Publication Nos. 3-49308 and 10-178330.
The conventional surface acoustic wave filters having an interdigital transducer disposed on a glass substrate often suffer the problem that the glass substrate cracks when lead terminals are soldered to terminal electrodes of the interdigital transducers.
In order to solve the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter which prevents the glass substrate from cracking when lead terminals are soldered to terminal electrodes of interdigital transducers.
According to one preferred embodiment of the present invention, a surface acoustic wave filter includes a glass substrate, an interdigital transducer provided on the glass substrate, a piezoelectric film arranged on the glass substrate so as to cover the interdigital transducer, a terminal electrode provided on the glass substrate and electrically connected to the interdigital transducer, and a coating film having an area smaller than that of the terminal electrode, the coating film being arranged on the terminal electrode such that a portion of a top surface of the terminal electrode is exposed throughout the periphery thereof.
It is preferable that the coating film has a better wettability with solder than the terminal electrode. For example, the terminal electrode preferably includes aluminum and the coating film preferably includes at least one of nickel and silver.
It is also preferable that an edge of the coating film is recessed from an edge of the terminal electrodes by about 10 xcexcm.
According to the surface acoustic wave filter of preferred embodiments of the present invention, the coating film does not directly contact the glass. Consequently, the stress caused by contraction of the solder acting on the coating film is absorbed by the terminal electrode to prevent the stress from directly acting on the glass substrate, thereby preventing cracks from being generated in the glass substrate.
The contraction stress acting on the glass substrate is reduced to about one-half of the contraction stress acting on the conventional glass substrate, when the periphery of the terminal electrode protrudes to the exterior by an amount that is about 10 xcexcm or more than the periphery of the coating film.
Other features, advantages, characteristics and elements of the present invention will become apparent from the detailed description of preferred embodiments below with reference to the attached drawings.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown